An accelerometer is a type of transducer that converts acceleration into electronic signals. Accelerometers are used in a wide variety of devices and for a wide variety of applications. For example, accelerometers are often included in various automobile systems, such as for air-bag deployment and roll-over detection. Accelerometers are often also included in many computer devices, such as for motion-based sensing (e.g., drop detection) and control (e.g., motion-based control for gaming).
Microelectromechanical systems (“MEMS,” also referred to as “MEMS devices”) are a specific type of integrated circuit used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a movable structure suspended above a substrate, and associated circuitry that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
Generally speaking, a MEMS accelerometer typically includes, among other things, a proof mass and one or more sensors for sensing movement or changes in position of the proof mass induced by external accelerations. Accelerometers can be configured to sense one, two, three, or even more axes of acceleration. Typically, the proof mass is configured in a predetermined device plane, and the axes of sensitivity are generally referred to with respect to this device plane. For example, accelerations sensed along an axis parallel to the device plane are typically referred to as X or Y axis accelerations, while accelerations sensed along an axis perpendicular to the device plane are typically referred to as Z axis accelerations. A single-axis accelerometer might be configured to detect just X or Y axis accelerations or just Z axis accelerations. A two-axis accelerometer might be configured to detect X and Y axis accelerations or might be configured to detect X and Z axis accelerations. A three-axis accelerometer might be configured to detect X, Y, and Z axis accelerations.
One category of Z-axis accelerometer uses a proof mass that is configured in a “tilt-mode” configuration, where the proof mass is supported from a substrate such that the proof mass rotates relative to the substrate under Z-axis acceleration. Sense electrodes placed below (e.g., on the underlying substrate) or both above and below the proof mass, which in many types of accelerometers are capacitively coupled with the proof mass, are used to sense such rotation of the proof mass and thereby to sense Z-axis acceleration.